FIG. 11 is a sectional view illustrating a prior art visible light laser diode. This visible light laser diode has an oscillation wavelength of 633 nm. In the figure, reference numeral 1 designates an n-type GaAs substrate. Using an MOCVD apparatus, an n-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2 having a thickness of 1.5 .mu.m, an undoped Ga.sub.0.5 In.sub.0.5 P active layer 6 having a multiple quantum well structure, a first p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3a having a thickness of 0.25 .mu.m, a Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 having a thickness of 5 nm, and a second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b having a thickness of 1.25 .mu.m are successively formed on the n-type GaAs substrate 1. After the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b is formed into a mesa shape by selective etching, using again the MOCVD apparatus, an n-type GaAs current blocking layer 4 is formed on a portion where the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b is etched and removed. Further, a p-type GaAs cap layer 5 is formed on the mesa shaped second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b and the n-type GaAs current blocking layer 4 to form a layer structure, and an n-type and p-type electrode 13 and 14 are formed on the lowermost and uppermost surfaces, respectively, of this layer structure.
A function of an etching stopper layer and a production method of a mesa shaped cladding layer will be described.
At first, in order for lateral mode control of the visible light laser diode, it is required to keep the first p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3a to a predetermined thickness, (in the case of illustrated prior art visible light laser diode structure, 0.25 .mu.m) and to form the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b in a mesa shape. In this example, however, because the same material, (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P, is used for both the first and second cladding layers, there is no difference in the etching rates of the layers. Therefore, it is impossible to form the two cladding layers into the above-described shape by forming the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b directly on the first p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3a.
Accordingly, an etching stopper layer having a predetermined thickness, comprising a material having an etching rate sufficiently lower than that of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P to the etchant used for forming, the mesa shape of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P, is introduced between the first and second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layers, whereby it is possible to form these layers into a cladding layer of the mesa shape having a skirt of a predetermined thickness. As shown in FIG. 12(a), when a sulfuric acid series etching solution is used, the above-described etching stopper layer 9, employing Ga.sub.0.5 In.sub.0.5 P having an etching rate which is around one-fiftieth of the etching rate of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P and having a thickness of 5 nm, is formed between the first and second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layers 3a and 3b.
When a SiN film 12 is used as a mask and the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b is etched with the above-described sulfuric acid series etching solution as shown in FIG. 12(b), by the function of the Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 having a sufficiently lower etching rate than the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b, it is possible to stop the etching with satisfactory controllability just before reaching the first p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3a and to form a cladding layer comprising the first p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3a having a predetermined thickness and the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b having a mesa shape on the undoped Ga.sub.0.5 In.sub.0.5 P active layer 6.
Subsequently, as shown in FIG. 12(c), an n-type GaAs current blocking layer 4 is formed where the second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b is etched and removed to form the mesa shape. After removing the SiN film 12 which was used as a mask in the above-described process of etching the cladding layer, a p-type GaAs cap layer 5 is formed on the exposed top surfaces of the mesa shaped second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 3b and the n-type GaAs current blocking layer 4, whereby a predetermined layer structure is formed.
Further, p-type and n-type electrodes 14 and 13 are formed on the upper surface of the p-type GaAs cap layer 5 and the lower surface of the n-type GaAs substrate 1, respectively, whereby a visible light laser diode as shown in FIG. 11 is obtained.
The visible light laser diode which is manufactured by the above-described method including the process of forming the mesa shaped cladding layer, is driven by applying a required driving current between the n-type and p-type electrodes 13 and 14 to emit a laser beam with a wavelength of 633 nm.
In the prior art visible light laser diode, GaAs is frequently used for the substrate, and generally an etching stopper layer is formed comprising Ga.sub.0.5 In.sub.0.5 P, lattice matching with the GaAs substrate to form a cladding layer on an active layer into a predetermined shape. Other than the above-described layer structure, Japanese Published Patent Application No. Sho 3-112186 discloses a visible light laser diode in which a buffer layer of Ga.sub.0.5 In.sub.0.5 P having a thickness of 40 .ANG. is introduced between two cladding layers of AlGaInP and AlGaAs in order to improve the voltage-current characteristic and to enhance the controllability of etching.
Japanese Published Patent Application No. Hei 3-73584 discloses a semiconductor laser device having an oscillation wavelength of below 660 nm, which is manufactured by a method including a process of successively forming a Ga.sub.0.5 In.sub.0.5 P etching stopper layer (with a thickness of 0.05 .mu.m) and a GaAs current blocking layer (with a thickness of 0.6 .mu.m) on a (Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer and a process of selectively etching a portion of the GaAs current blocking layer.
The prior art visible light laser diode and the manufacturing method thereof are as described above. In order to form the first and second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layers 3a and 3b on the undoped Ga.sub.0.5 In.sub.0.5 P active layer 6 into a predetermined shape, it is unavoidably required to introduce the Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 between the two cladding layers 3a and 3b. In addition, it is not possible to remove the Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 after forming the first and second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layers 3a and 3b into predetermined shapes, and a layer structure in which the Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 partly remains above the undoped Ga.sub.0.5 In.sub.0.5 P active layer 6 results, as shown in FIG. 11. Since the Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 has a thickness of 5 nm, a quantum well is formed between the first and second p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layers 3a and 3b, and the energy level of this quantum well is in the vicinity of 640 nm in wavelength. During forming (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P barrier layers 3d and 3e and a Ga.sub.0.5 In.sub.0.5 P well layer 9b and varying the well width of the Ga.sub.0.5 In.sub.0.5 P well layer 9b, the wavelength of photoluminescent (PL) light as measured at 4.2.degree. K is converted into a band gap energy and plotted by white circles as shown in FIG. 13(a), while solid line 1 shown in FIG. 13(b) shows the energy band gap as a theoretical value obtained from calculation of wave functions, and these represent the above-described results.
Since the above-described Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 having a thickness of 5 nm serves as a kind of absorption layer to light of an oscillation wavelength of 633 nm which is emitted from the undoped Ga.sub.0.5 In.sub.0.5 P active layer 6, it is a light sink and the current characteristic is deteriorated at high output, degrading the performance of the visible light laser diode. When the oscillation wavelength of the visible light laser diode is in the 670 nm band, the layer structure with the remaining Ga.sub.0.5 In.sub.0.5 P etching stopper layer 9 absorbs no light and causes no problem. However, recent development of the crystal growth technique enables utilization of the quantum effect which occurs due to reducing the active layer well width, thereby to shorten the oscillation wavelength in a visible laser diode. For example, a visible light laser diode replacing a HeNe Gas laser having a wavelength of 633 nm, having an active layer of a well width below 4 nm has been described in various papers presented before Societies, for instance, Japanese Journal of Applied Physics, Autumn of 1992. The shortening of the oscillation wavelength of such visible light laser diodes is required for application to optical disks, bar code readers, laser beam printers, or the like, as a trend of the times. This enables improvement in the speed of data writing into an optical disk, reduction in the required power, or the like. This further eases data recorded in a high vision system, further advancing the shortening of wavelength, hereby making the above-described problems unavoidable ones.
Japanese Journal of Applied Physics, Vol. 29, No. 9, September, 1990, 11.L1669-L1671, discloses a 632.7 nm visible light laser diode, which comprises a p-type (Al.sub.0.19 Ga.sub.0.81).sub.0.5 In.sub.0.5 P etching stopper layer (with a thickness of 5 nm) disposed on a p-type (Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P inner cladding layer (with a thickness of 0.25 .mu.m) disposed on an undoped (Al.sub.0.19 Ga.sub.0.81).sub.0.5 In.sub.0.5 P active layer, and a p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P outer cladding layer (with a thickness of 0.7 .mu.m) formed into a mesa-shape. Though this layer structure is free from light absorption by the etching stopper layer, when the p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P outer cladding layer is formed into the mesa shape, the p-type (Al.sub.0.19 Ga.sub.0.81).sub.0.5 In.sub.0.5 P etching stopper layer including 19% of Al has a small etching selectivity ratio relative to the p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P outer cladding layer, and it was impossible to stop the etching sufficiently.